The basal forebrain (BF) is a complex brain region that plays an important role in modulating cortical activity. The BF contains cholinergic and various non-cholinergic corticopetal neurons and interneurons. The corticopetal BF projections, especially the cholinergic component, have been implicated in memory, sensory processing and attention. In spite of intensive efforts by many laboratories over the last two decades, it remains enigmatic how the BF is organized to support both general arousal as well as specific functions like attention. Novel 3D reconstructions and numerical analyses suggest that the distribution of the various cell types is not random but displays a general pattern of association. Within the cholinergic space (i.e. the volume occupied by the cell bodies of cortically projecting BF cholinergic neurons) different cell types occupy high- density cell clusters that are regionally specific. It is hypothesized that these cell groups (clusters) in various locations in the BF together with specific prefrontal and posterior cortical areas may provide the neural basis of a distributed functional network that orchestrates localized cortical modulation. Specific projects in this application are variants on the theme of anatomical segregation of functional domains in basalo-cortical networks. Specific Aim 1 will define and validate BF cell clusters across individuals of rats and will assess the intersubejct variability of clusters. Specific Aim 2 will define the cortical projection target of BF cell clusters. Specific Aim 3 will test the hypothesis that the BF is topographically organized such that specific prefrontal cortical areas target specific BF cell groups, which in turn innervate specific limbic, posterior sensory or associational cortical areas. Specific Aim 4 we will characterize the biophysical and anatomical properties of local processing with special reference to NPY-cholinergic interaction, using in vitro paired recordings with intrinsic (GFP-NPY) labeling for NPY neurons and in vivo (Cy3-192IgG) pre-labeling for cholinergic neurons. We hypothesize that NPY neurons suppress cholinergic firing. The data acquisition in these projects represents a complete transition from cell populations to electron microscopy reconstructions of single synapses and investigations of their functions. The project will employ highly innovative approaches to data analysis, that bridge several important methodological gaps relating to cross-scale integration of neuroanatomical data collected at the cellular and systems levels. The proposed study will lead to more realistic description of basalo-cortical networks at the brain-wide scale that can guide and constrain behavioral studies on cholinergic function, in particular mechanisms of sensory integration and attention. Concomitantly, it will facilitate the understanding of the aberrant processing in basalo-cortical networks that characterizes several neuropsychiatric disorders, including Alzheimer's disease, schizophrenia, and drug abuse.